This invention relates to ring laser gyroscopes in general and more particularly to an improved system for eliminating lockin in a ring laser gyroscope.
Various types of ring laser gyroscope apparatus have been developed. Typical is the apparatus disclosed in U.S. Pat. No. 3,373,650. As explained therein, in a ring laser gyroscope, two monochromatic beams of light are generated in two opposite directions around a closed loop path about the axis of rotation. A rotation of the apparatus about that axis causes the effective path length for the two beams to change to produce a frequency difference between the two beams since the frequency of oscillation of a laser is dependent upon the length of a lasing path. It is possible, by combining the two waves, to generate interference patterns and from these patterns to obtain a measure of the rotational rate about the axis.
However, as explained in this patent, at low rotational rates, when the difference in frequency between the two beams is small, they tend to resonate together or lock-in and oscillate at only one frequency. As a result, low rotation rates become impossible to read. The device of U.S. Pat. No. 3,373,650, overcomes this problem by oscillating or dithering the apparatus so as to avoid lock-in of the two beams. A further apparatus of this nature is disclosed in U.S. Pat. No. 3,467,472. A detailed explanation of the problem and the various proposed solutions thereto, is contained in U.S. Pat. No. 3,879,130. The system disclosed therein takes a different approach to the problem using a saturable absorber internal to the ring laser cavity as a solution to this problem. The present invention relates basically to the type of solution proposed by U.S. Pat. Nos. 3,373,650 and 3,467,472, i.e., dithering. In all mechanical dithering systems to date, operation has been open loop. Although lock-in is avoided to a large degree, a certain amount of residual lock-in remains present and causes errors which are often unacceptably large. Similar problems occur when using optical dithering or some other type which has the same effect.
Thus, the need for a system which produces a dither in which the residual lock-in effect is absent or much smaller than in previous dither producing techniques, becomes evident.